THE CAUSATION OF THE HEART-BEAT 



959 



the pressure in its interior must rise at each heart-beat above the maximum 

 systolic pressure in the aorta, e.g. to 110 mm. Hg. The aortic valves will 

 open as soon as the pressure rises above 60 mm. Hg. The arterial resistance 

 is now increased so as to raise the average pressure to 120 mm. Hg. The 

 heart now may raise the pressure in its interior to 120 mm. Hg. This will 

 be higher than the diastolic pressure 

 in the aorta and a certain amount 

 of blood will escape, but the outflow 

 of blood will cease as soon as the 

 pressure in the aorta is equal to 

 that in the ventricle. Diastole will 

 then occur, the ventricle will relax 

 before it has emptied out 10 c.c. of 

 blood. Let us assume it has forced 

 out 3 c.c. of blood it will then 

 contain an excess of 7 c.c. of blodd 

 at the end of diastole. Meanwhile 

 the venous inflow is proceeding at 

 the same rate as before so that at 

 the end of diastole it has 7 c.c. more 

 blood than it had at the end of 

 the previous beat, i.e. its diastolic 

 volume will be increased and the 

 heart will be dilated. At the in- 

 creased beat we find that the con- 

 traction of the ventricle is much 

 more forcible. The maximum pres- 

 sure now rises to 130 mm. Hg. and 



8 c.c. of blood are sent out into 

 the aorta. At the end of this beat 

 the heart will be still fuller than 

 before, containing an excess of 



9 c.c. of blood. The third beat 

 is still more forcible, the intra- 

 ventricular pressure rising to a 

 maximum of 140 mm. Hg., and 



10 c.c. of blood are expelled. After this the heart goes on beating regularly 

 expelling 10 c.c. of blood at each beat, i.e. the same amount as it receives 

 from the veins, and the arterial pressure is maintained constant at an average 

 of 120 mm. Hg. But the heart remains more dilated than it was previously 

 since it contains an excess of 9 c.c. of blood. If now the arterial resistance 

 be suddenly reduced to its previous amount, the first beat after the change 

 may send out 17 c.c. of blood, the second beat 12 c.c. of blood and the third 

 beat JO c.c. as before. We _see therefore that the energy set free at each 

 contraction of the heart is increased by increasing the volume of the heart, 

 but increased volume of the heart means increased length of the muscular 



FIG. 438. Effect 01 increased arterial pres- 

 sure on the volume changes of the heart, 

 with a steady inflow of 154 c.c. blood per 

 10 seconds. 



C. cardiometer curve. B.P. arterial blood- 

 pressure. V.P. pressure in the inferior 

 vena oava. The lines 100 and 80 show the 

 height of the blood- pressure in mm. Hg. 



